(1) Field of the Invention
This invention relates to an impactive energy absorbing structure to be attached to the boss of a steering wheel in an automobile, and more particularly to an impactive energy absorbing structure consisting of an energy absorbing member of a brittle plastic material and a cover member.
(2) Description of the Prior Art
As an impactive energy absorbing structure for attachment to the boss of a steering wheel, there has been proposed an arrangement which, as illustrated in cross section in FIG. 1, comprises an energy absorbing member 2 molded of a brittle plastic material in a solid structure of prescribed shape, an insert 3 adapted to embrace the energy absorbing structure 2 and retain the external shape thereof, and a cover member 4 of a flexible plastic material serving to envelop the insert 3.
Generally, the energy absorption efficiency of the impactive energy absorbing structure is calculated by the formula, S/(Lmax X Dmax) (wherein S denotes the amount of energy absorbed, Lmax the value of the maximum allowale load, and Dmax the amount of the maximum deformation). Since the value of the maximum allowable load, Lmax, and the amount of the maximum deformation, Dmax, are fixed in advance by the shape and other specification of the structure, the energy absorption efficiency increases in proportion as the amount of energy absorbed, S, increases.
The energy absorption characteristic of the cover member 4 of the conventional impactive energy absorbing structure is as indicated by the curve b and that of the energy absorbing member 2 of the same structure is as indicated by the curve c respective shown in the graph of FIG. 2. The energy absorption characteristic of the aforementioned structure as a whole is represented by the curve a which is the sum of the curve b and the curve c. The amount of energy absorbed, S, in this case, therefore, falls in the area S enclosed with the curve a and the x axis. Thus, because of the blank part which occurs between the curve a and the line of the maximum allowable load, Lmax, the conventional impactive energy absorbing structure is not allowed to increase the amount of energy absorption, S, to the maximum. It has suffered from the problem of insufficient energy absorption efficiency.